1. Field
The present application relates to semiconductor devices typically used in power electronics, such as an IGBT (Insulated Gate Bipolar Transistor) or a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
2. Related Art
The IGBT is currently the preferred solid state power amplifier in the power industry. It competes for power switching functions with the power MOSFET. The IGBT combines the high power capability of the bipolar transistor with the voltage control capability of the MOSFET. The IGBT is typically used in high voltage power applications where the peak voltage VCE across the device is greater than 1000 V and the device supports output powers of greater than 10 kW. The IV characteristics of the IGBT typically exhibit high current density controlled by a relatively small voltage. The output resistance is much higher than the common emitter configuration of the bipolar transistor, i.e. Rout is more like the common base configuration and the dc input resistance is infinite due to its insulated gate. Thus, the device displays high transconductance in which the output current is controlled by the input voltage. In general, high voltage, high current and low switching frequencies favor IGBTs while low voltage, low current and high switching frequencies are the domain of the power MOSFET.
A conventional prior art vertical type n-channel IGBT is shown in FIG. 1, which includes an N− base layer 103 formed on a P+ substrate 101. The N− base layer 103 forms an epitaxial drift region. A P+ body region 105 is formed by diffusion into the N− base layer 103. An N+ emitter region 107 is formed by diffusion into the P+ body region 105. A gate electrode 111 is formed on an insulating layer 109 that covers a central portion of the N+ emitter region 107, a central portion of the P+ body region 105 and a central portion of the N− base layer 103 as shown. The insulating layer 109 is patterned to expose an outside portion of the N+ emitter region 107 and an outside portion of the P+ body region 105. An emitter electrode 113 is formed in ohmic-contact with the exposed outside portions of the N+ emitter region 107 and an outside portion of the P+ body region 105 as shown. A backside collector electrode 115 is disposed in ohmic-contact with the P+ substrate 101.
The IGBT functions in an ON (conducting) state where current conducts vertically between the top emitter electrode 113 and the backside collector electrode 115 by the following operations. Specifically, a positive bias voltage is applied between the collector electrode 115 and the emitter electrode 113 and a positive bias voltage is applied to the gate electrode 111 relative to the emitter electrode 109 such that inversion layer of electrons forms under the gate electrode 111 at or near the interface between the central portion of the P+ body region 105 and the insulating layer 109. In this manner, electrons are injected from the N+ emitter region 107 into the N− drift region 103. In accordance with the injection amount of the electrons, holes are injected from the collector 115 through the P+ substrate 101 to the N− drift region 103. As a result, the N− drift region 103 is filled with carriers, which decreases the effective resistance of the N− drift region 103 brings the IGBT into the ON (conducting) state.
On the other hand, the IGBT functions in an OFF (non-conducting) state where there is little or no current that conducts vertically between the top emitter electrode 113 and the backside collector electrode 115 by the following operations. Specifically, a reverse bias voltage is applied to the gate electrode 111 relative to the emitter electrode 109 such that there is no inversion layer of electrons formed under the gate electrode 111 at or near the interface between the central portion of the P+ body region 105 and the insulating layer 109. As a result, electrons are not injected from the N+ emitter region 107 into the N− drift region 103, and holes are not injected from the collector 115 through the P+ substrate 101 to the N− drift region 103. As a result, the N− drift region 103 is depleted of carriers, which increases the effective resistance of the N− drift region 103 brings the IGBT into the OFF (non-conducting) state.
When configuring the device from the ON state to the OFF state, holes accumulated in the N− drift region 103 are exhausted through the P+ body region 105 and to the emitter electrode 113. This flow of accumulated holes can limit the switching frequency of the IGBT and thus adversely impact the suitability of the IGBT for applications where high switching frequencies are required. For example, typical IGBTs operate at switching frequencies less than 100 kHz.
Gate drive circuitry that control the ON-OFF operation of one or more IGBTs are also commonplace. Such gate drive circuitry can include one or more opto-isolators that employ a light emitting diode and corresponding photodiode that interface to one another via optical signals emitted by the light emitting diode and received by the corresponding photodiode. The opto-isolator provides for electrical isolation between an electrical input signal and the electrical gate drive signal that controls the ON-OFF operation of the IGBT.
US2012/0098029 describes a thyristor core with an optically-activated gate that is controlled by monolithically or hybrid integrated switches S1 and S2 and a laser for device turn-on and off.